Steam seen at Fukushima reactor

18 July 2013

Investigations are underway into the source of what appeared to be steam rising from the fifth floor of the damaged reactor building of unit 3 at the Fukushima Daiichi plant. Rainwater entering the reactor well is considered the likely cause.

The steam was first noticed at around 8.20am. Monitoring systems indicated no changes in the temperature and pressure within the reactor pressure vessel or the primary containment vessel (PCV). The nitrogen injection system into the PVC was found to be operating normally, as was the water cooling systems for both the reactor and the used fuel pool. No increase in radiation or dust levels was detected.

Plant owner Tokyo Electric Power Company (Tepco) said that subsequent monitoring of the reactor building had also not identified any changes in the parameters.

However, Tepco said, "We assume that the steam was generated as a result of rain leaking through gaps near the cover [of the reactor well] and being heated at the head of the primary containment vessel."

A worker at the plant spotted "steam-like gas wafting through the air" whilst using a remotely operated camera to survey the fifth floor of the building prior to debris removal work. The vapour appeared to be rising from the central area of the fifth floor near the pool used for storing the reactor's steam dryer and moisture separator during maintenance and refuelling operations. This pool is separate from the used fuel storage pool, which is located on the other side of the reactor well.

Tepco said that it would continue monitoring conditions and investigate the source of the steam.